Human parathyroid hormone, or hPTH, is a protein product of the parathyroid glands that exerts various physiological effects. Of particular clinical significance is its anabolic effect on bone tissue, and the possibility that PTH therapy can slow or reverse the progression of osteoporosis, and be of benefit in the treatment of related bone disorders.
As a prelude to using hPTH as a pharmaceutical product, it is necessary for many reasons to provide the protein in essentially pure form. For instance, the use in clinical trials of PTH that is essentially pure will permit observed effects to be attributed solely to PTH and not to some structurally related contaminant. Also, obtaining hPTH in essentially pure form will provide for greater specific activity i.e. highest potency per unit amount of hPTH, and permit administration of the smallest possible dosage size to treat a given indication. Furthermore, removal of contaminants will effectively reduce the possibility of side effects, which is especially important when PTH is used to treat chronic ailments such as osteoporosis.
One method currently used to purify proteins, and to analyze protein purity, is reversed phase high performance liquid chromatography (RP-HPLC). Like other HPLC techniques, the reversed phase approach exploits variability in the rates at which specific proteins migrate through a bed of silica microspheres. In the reversed phase HPLC technique, however, alkylated silica microspheres are used, and migrating proteins are subjected to a two phase solvent system which exploits protein charge and accents separation. Most typically, the solvent system consists of a water phase and an organic phase typically containing acetonitrile and an ion-pairing agent (also known as a charge modifier) such as trifluoroacetic acid (TFA), the relative proportions of which are altered gradiently by automated blending as the protein sample migrates through the column. When analyzed by this technique, a protein preparation which elicits but a single detectable protein species (measured by UV absorbance either at 214 nm or at 280 nm) is deemed to consist of one protein species, and is thus characterized as an essentially pure protein. Proteins exhibiting this degree of purity are sometimes characterized as being of "HPLC-grade".